Use this topic to post any helpful/interesting undocumented/hard to find out about (buried in forums and such) preferences so that a dev can easily update the preferences wiki page with a bunch of settings.

kmeleon.display.toolbars_line False=Hides the lines between toolbars.
kmeleon.display.lockedToolbarAlt True=Alternate spacing when toolbars are locked.

No the second has nothing to do with lines.
It just makes that when *locking* toolbars, the buttons stay exactly where they are, meaning there'll be a little empty space on the left side of each toolbars, where that drag-handle was. Without that setting, this little empty space will be on the right side of toolbars, all buttons moving a tiny little bit to the left ;-) At least that is what I noticed when testing it awhile ago

You must create a new pref entry in about:config.
Go to Edit -> Configuration -> Browser Configuration (or type about:config in URL bar)
Right click in preference list, and select New -> Boolean.
Named it: font.askWhenNeeded
And set value to: false

How do to have a close button (X) in each tab?

This feature is included since 1.5.1 version. You must create a new pref entry in about:config.
Go to Edit -> Configuration -> Browser Configuration (or type about:config in URL bar)
Right click in preference list, and select New -> Integer.
Named it: browser.tabs.closeButtons
And set value to: 1
Restart K-Meleon to changes take effect.

QuoteBugs
About bug # 1046: Bottom tab bar not hidden in fullscreen mode
Dorian said:
"Forgot to say that for bug 1046 I've added:
kmeleon.plugins.fullscreen.hide_tabsbar
which is only relevant for fixed and bottom tabs bar."

Another a lot of used are:

dom.max_script_run_time <integer> => Timeout in seconds to prompt for stop a running Javascript.

browser.chrome.favicons <boolean> => Search favicon file in web domain to show the favicon if it isn't declarated in web page code.